VirtualFileSystem: Add YAMLVFSWriter to generate VFS mapping files

This moves the logic to write a JSON VFS mapping from the C api into
VirtualFileSystem, so that we can use it internally.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209241 91177308-0d34-0410-b5e6-96231b3b80d8